This class is used by the Graphics2D SVG Generator to manage
a group of Nodes that can later be added to the SVG DOM Tree
managed by the DOMTreeManager.
There are two rules that control how children nodes are
added to the group managed by this class:
+ Children node are added to the group as long as
there is no more than n graphic context overrides needed to
describe the children style. A graphic context override
happens when style attributes need to be added to a child
node to reflect the state of the graphic context at the
time the child was added. Note that the opacity is never
reflected in a group node and therefore, is not accounted
for in the number of overrides. The number of overrides can
be configured and defaults to 2.
+ Children nodes are added to the current group as long as
the associated GraphicContext's transform stack is valid.
When children nodes can no longer be added, the group is considered
complete and the associated DOMTreeManager is notified of the
availability of a completed group. Then, a new group is started.
The DOMTreeManager is also notified every thime a new element
is added to the current group. This is needed to let the
DOMTreeManager handle group managers that would be used concurrently.